I looked at some of the Magpul videos where their two instructor guys are using them. They are laying their thumb parallel to the barrel, extended forward and resting on the fore grip. I think they are going for grip consistency since it looks similar to a thumbs forward position with a pistol.

I don't think it's any better or worse based on what I saw in the videos and "looks" a bit strange to me however if it works for them or someone else, more power to them.